Fibreglass Components

For mirror cells, tube bases etc.

Fibreglass (GRP - Glass Reinforced Plastic) seems to be a little-used material amongst ATMers. Perhaps for good reason as it is a pretty messy, smelly job. However, it is extremely strong, light and the finished parts are easy to work and can be given a good finish.

I first came across the idea in Howards' Standard Handbook of Telescope Making. There he describes the use of a mold (for a mirror cell) constructed by making an impression of a wooden pattern in plaster of paris. I tried this but found it very difficult to remove the pattern without damaging the mold excessively. So I came up with the following idea, which works quite well. The description here details parts made for my 8in Newtonian.

Materials:

Tools:

1. Making the molds.

This is really best done with a router. The required pattern is drawn out on a piece of MDF and the router used to cut the pattern out to a depth of about 6mm (or greater for larger components). For circular parts like these the trammel point can be used, or a simple circle cutting jig could me be made. The straight lines can then be done by hand or using a piece of wood tacked to the mold as a guide. When all the lines have been done the spaces in between can be routed out freehand. If a router is not available then it would be possible to make molds by cutting out sections of MDF and screwing them down to another piece of wood to create the same result. The molds in the photo are for a tube end-ring, tube base and cell base.

MDF molds for a tube end-ring, tube base and cell base

2. Casting.

SAFETY NOTE: work in a well ventilated area.

When casting it is important to have everything to hand as you have to work quickly because the resin goes off quite rapidly. The kit contains fibreglass mat, resin and hardener and a little measuring jug, brush and sticks for mixing. A useful tip is to replace the polythene gloves supplied with tighter fitting latex ones (available form DIY stores) as the former tend to become a hindrance as they get sticky from the resin.

The mat should be cut in sections to fill the mold, straight pieces and arcs etc. Don't bother trying to cut out the complete shapes as single pieces as they prove difficult to work with. You'll need about seven or eight layers to fill a mold 6mm deep. For making tube rings you need to just try and work with thin strips of mat. To prevent the resin sticking to the mold it should be smeared with a thin layer of petroleum jelly, making sure to wipe out any excess with a rag. With everything to hand the resin can be mixed, according to the instructions. Then paint a liberal layer into the mold and then apply a layer of mat. Then continue building up layers of resin and mat alternately until you reach the surface of the mold. You will need to mix more resin as you go as the mold volume takes a fair bit, I found I could work with volumes of 60ml.  Set aside to dry, and don't worry that it looks a complete mess at this stage.

3. Releasing the part.

SAFETY NOTE: when sanding fibreglass always wear a mask.

Before the part is released from the mold some finishing should be done. This is where the electric sander comes in using some coarse sandpaper. Work all over the exposed area of the part until it is smooth and flush with mold surface.

To release, use a saw to cut in several places all around the mold and right up to the part, then the mold can simply be broken away. This actually works quite well, though you might need to prise some bits of the mold away with a bit of force.

Given the sanding that has already been done the part is fairly clean on release. If there are rough bits they can be sanded off or cut away with a sharp knife.

4. Final finishing.

An electric detail sander is very useful here, though it could be done by hand. Fibreglass does tend to be a little pitted due to airbubbles in the resin, though this is largely covered by adequate priming and painting. If there are larger blemishes a car body type filler can be applied.

This photo shows the finished cell for an 8in mirror, the base having been painted matt black. This is much simpler than that described in Howard. Instead of a continuous ring around the edge, this design just uses three small metal angle brackets screwed to the base and tapped to take small bolts for centering the mirror. In addition small pieces of angle aluminium were attached to the top of the brackets as mirror retaining clips. They are held by cutting a slot in them which slips over the centering bolt, which takes a small nut for tightening against the clip. Three long M6 bolts pass through the cell base and are used for alignment by sliding short springs over the bolts which then pass through holes in the tube base and take wing nuts for adjustment. To the top of the bolts small ball bearings have been glued for the mirror to sit on.

This last picture shows the finished tube base and end ring, spray painted with a light green enamel.

Good luck if you have a go at this technique.